ABSORPTION OF METHYLBACTERIOPHEOPHORBIDE a SINGLE CRYSTALS: SPECTRAL SHIFTS DUE TO π‐π INTERACTIONS

— Polarized absorption spectra were obtained for a single crystal of methylbacteriophorbide a (MeBPhide a). The Qy band is red-shifted ? 1660 cm-1 (? 110 nm) relative to MeBPhide a in a CH2Cl2/benzene solution. This is equivalent to the largest red shifts observed for in vivo bacteriochlorophyll a. The Soret band exhibits a smaller red shift and a significant reduction in intensity, and the Qx band is not observed. The crystal spectra are qualitatively similar to spectra reported for several other aggregated (bacterio)chlorophyll and bacteriopheophytin systems. Since crystalline MeBPhide a contains no Mg, water or hydrogen bonding (Barkigia etal., 1981), these results demonstrate that the spectral changes associated with the aggregation of photosynthetic chromophores can arise solely from IT-IT interactions between macrocycles.